1. Field of the Invention
This invention relates to a process for producing a mold for casting using a slurry of refractory particles and colloidal alumina as binder. More particularly it relates to a process for producing a mold suitable for use as a core used in a precise casting mold cavity.
2. Description of the Prior Art
In making molds, particularly cores used in molds, there are used in general water glass, clays, plastics and the like as the binder. These binders have considerable binding strength and are stable at low temperatures, but their binding strength is lowered at high temperatures at 1200.degree. C. or higher. Particularly when a fluid metal at high temperature is cast, there arises a defect in that the core is deformed and sometimes destroyed.
As a process for producing a heat resistant core, the use of a solution of hydrolyzed ethyl silicate as a binder is disclosed, for example, in Japanese Patent Appln Kokoku (Post-Exam Publn) No. 20848/63. According to this process, a hydrolyzed solution of ethyl silicate is mixed with refractory particles to prepare a slurry, which is filled in a pattern and gelled, followed by drying and firing. In order to proceed gelling, the pattern is sometimes dipped in water. Since an alcohol is used as binder and should be evaporated with the progress of gellation, there easily take place fine cracks in the mold in such a time. Thus, the strength of the mold is lowered by the generation of cracks.
In another process for producing a heat resistant mold, the use of moisture-containing colloidal alumina as the binder is disclosed, for example, in Japanese Patent Appln Kokoku (Post-Exam Publn) No. 32822/70. According to this process, silica sand particles coated with moisture-containing colloidal alumina are used as the molding material and filled in a pattern (or a core box) and compacted, followed by drying and firing after removal from the pattern. According to this process, fluidity of the sand is worse and no mold with high dimensional accuracy is obtained since compacting is conducted for producing the mold. Therefore, this process cannot be applied to cores with complicated shapes. Further, since compacting is necessary, the refractory particles should be coarse ones, which results in making the mold surface rough. Further, there arises another problem in that the packing density of the refractory particles changes at random in portions of the mold. In this process, if the water content increases in order to improve the fluidity of the sand, deformation of the mold takes place at the time of drying or firing.